1. Field Of The Invention
This invention relates to an improved means at the beginning of the tension section for automatically establishing a normal range of tensions in that section under regular operating conditions and for promptly decreasing that tension for machine shutdown purposes under thread-breakage conditions.
2. Description Of The Prior Art
In many multi-needle machines, which are equipped with stop motion means, the individual threads coming from each supply spool are all wrapped around a common shaft or tensioning roll, located at the beginning of the thread-tensioning section. Each thread is then directed successively through (1) a tension-holding frictional device, which is located at the end of the thread-tensioning section, and (2) a take-up bar, which is located between the thread-tensioning section and the needles. The take-up bar is arranged to pull thread from each spool during the upstroke of the needles in order to have a supply of thread available for use by the needles during their ensuing downstroke. The pull of the take-up bar causes the threads to rotate the common wrap-around roll, which is lightly braked to an adjustable degree so as to resist the pull sufficiently to build up a desired degree of tension on the threads in the thread-tensioning section.
The tension of each individual thread in that section (and the speed at which it moves through that section) will vary because all such threads are not used or consumed in the sewing operation at the same rate; hence, they are not withdrawn from the thread-tensioning section at the same rate. The most rapidly consumed threads move the fastest and exert the highest pull on the common wrap-around roll. The least rapidly consumed threads move the slowest and exert the lowest pull. Since all threads, fast and slow, enter the thread-tensioning section at the same high speed, which is dictated by the fastest threads, and since they leave that section at different rates ranging from high to low, the tension of the fast threads is high while that of the slow threads is low. As a consequence, it is not uncommon for a given fast thread to be overloaded to its breakage point or for the tension on a given slow thread to drop below a desired minimum due to breakage. However, when this below-minimum tension drop occurs in an unbroken thread having a proper usage rate, it may be falsely sensed by the sag sensor as a broken thread; hence, the sag sensor responds by shutting down the machine. This false indication of breakage is highly objectionable. Where the drop is due to the usage of the slowest thread at a slow rate below the desired minimum, a shut-down is desirable. See U.S. Pat. Nos. to Hangartner 2,696,608, Kuhn 3,009,433, Vossen 3,094,855 and Jackson 3,529,560.
In our prior U.S. Pat. No. 3,698,335 granted Oct. 17, 1972, we omit the common wrap-around thread-driven shaft and substitute a thread-tensioning mechanism comprising: a power-driven take-over shaft; a group of independent wrap-around wheels, one normally driven by each thread; one-way clutch means mounting the wheels on the take-over shaft; and power means for driving the common shaft at a constant take-over speed somewhat below the lowest wheel speed corresponding to the lowest rate of thread usage. A given thread, which is wrapped around its wheel, normally functions to drive its wheel at the corresponding rate of usage. As the normal rate of usage for that given thread rises and falls: its pull or tension rises and falls; and its wheel speed rises and falls within an operative range above the shaft take-over speed. If the tension becomes too high, the thread breaks, the wheel speed drops and the take-over shaft becomes operative to drive the thread at the take-over speed. If the tension of an unbroken thread becomes too low, once again the speed of the thread-driven wheel drops and the take-over shaft becomes operative. In either case, the drop in sag renders the conventional tension sensor operative to shut down the machine.